Imaging of radiation has applications ranging from medical imaging to geophysical mapping. It is often desirable to image a radioactive source using a fibre optic camera. Imaging with a fibre optic camera allows the imaging device to be more compact, flexible and easier to handle, and allows the signal to be relayed to a shielded high-performance camera. However, the use of fibre optics in proximity to a radiation source creates noise artefacts due to the radiation interacting with the glass in the fibre optics, in particular scintillation, Bremsstrahlung and Cerenkov radiation. When imaging weak sources of radiation, these noise artefacts can completely obscure the signal of interest.
One common source of radiation is diagnostic radiopharmaceuticals used for medical imaging, such as Flurodeoxyglucose ([18F]-FDG). A sample containing [18F]-FDG emits beta+ particles (positrons) that subsequently annihilate with electrons to produce 511 keV gamma particles. In the above example, discriminating beta particles from the large gamma ray background is exceedingly difficult. The ability to accurately detect and image fluorine-18 beta particles would allow for greatly improved disease detection and treatment.
Typically, radiation is detected with the use of a material which is design to emit photons when excited by ionising radiation. Such a material is known as a ‘scintillator’. A scintillator produces a signal which is (statistically) proportional to the energy absorbed from the incident radiation. Therefore, as the absorbed energy increases, more photons are produced.
Scintillators are normally combined with an optical detector capable of converting photons into electrons. Suitable devices include silicon photomultipliers, photomultiplier tubes and charge coupled devices (CCD).
Known methods of discriminating between different types of scintillation rely on pulse discrimination. The signal produced by the group of photons emitted from a single ionised particle/scintillator interaction is known as a ‘pulse’. Two components of the pulse measured by a detector are commonly measured, the ‘pulse-height’ and the ‘pulse-shape’. Energy differences between ionising particles gives rise to characteristic pulse-heights and pulse shapes.
One disadvantage with these methods however is that, as the radiation flux increases, the number of pulses produced per second increases, which reduces the average time between each such pulse. If the temporal resolution of the detector is insufficient to capture individual pulses, it can be impossible to discriminate between interaction events.
In order to mitigate against this problem scintillators with very fast temporal responses, and which have a very low after-glow, are used. Alternatively, the number of pixels of the optical detector can be increased, which reduces the number of pulses received at any given pixel per second. Both of these solutions are however not ideally suited for weak radioactive sources, as they tend to have poor optical characteristics and/or increased read noise.
Furthermore, intensive data processing is required to attempt to improve the signal/noise ratio; such attempts usually provide little gain. For these reasons practical devices for medical use have not been forthcoming.
Therefore, there exists a need to improve the detection of radiation. In particular, there exists a need for a practical detector capable of discriminating the signal from a weak form of radiation from within a relatively strong noise background. Such improvements to beta particle imaging would allow for a practical hand-held molecular imaging device.